Those of ordinary skill in the art relating to fluid control systems, for example automobile automatic transmission systems, know that ring seals are used to prevent leakage around a cylindrically shaped element coaxially mounted in a cylindrical chamber. In the example of an automobile automatic transmission, ring seals are mounted coaxially on piston heads and around piston shafts to prevent leakage of oil in the space respectively between the piston head and the cylindrical chamber in which it is mounted and between the shaft and the cylindrical wall of the opening through which the shaft extends.
The term "ring seal" is used herein generically to describe any seal that is circumferential, that has an inner and an outer diameter, that has a width between the inner and outer diameters, and that has an axial depth. Such ring seals are made of a variety of materials. A material is chosen to allow a seal to radially expand and withstand the temperature of a particular fluid in a cylinder, to slide easily while in intimate contact with the wall of the cylinder, and to endure the mechanical and chemical assault of the pressurized fluid. Particularly adapted to these ends for use in automatic transmissions are polytetrafluoroethylene, the material being known under the trademark "TEFLON."
No matter how durable a ring seal made of a particular material is for a particular use, the seal is susceptible to wear and failure and must be replaced from time to time. To avoid an unnecessary breakdown of the mechanical parts when replacing a ring seal (or even to facilitate the initial assembly of the mechanical parts), it is advantageous to have a seal separate along its circumference, so that the ring seal may be transversely placed into position over the element on which it is to be coaxially mounted.
A conventional means of fabricating ring seals that separate on their circumferences is to cut ring seal that are first manufactured with continuous circumferences. Two methods are employed. A first method is schematically illustrated in FIG. 1 of the drawings. The ring seal 10, before it is cut is circumferentially continuous. Blade 12, shown in two positions as blade 12a and blade 12b, is plunged directly into the seal 10. Blade 12a represents blade 12 before it is plunged into seal 10 or after it is withdrawn from seal 10. Blade 12b represents the blade after it is plunged into seal 10 but not yet withdrawn. Cutting the seal thus provides two cut surfaces, both of which are represented by the cutting plane 14 which is cross-hatched in the drawing. These surfaces are chamfered, mirror images of one another. The surfaces may be separated, owing to the preferably flexibility of the ring seal 10, so that ring seal 10 may be opened and transversely mounted onto a coaxial cylindrical element. The chamfered ends are placed into intimate contact with one another to keep the integrity of the seal around the complete circumference. Theoretically the matching surfaces 14 of the cut ends are in intimate contact when the seal is in place on or around the coaxial element. But contrary to the theory, the contact between the two surfaces is not as intimate as would be preferred. With use of the plunging method, the cutting tool 12a, 12b compresses the material ahead of it and puts the material still further ahead in tension. The material in tension tears as the cutting tool breaks through. The tearing does not provide for a uniform cut across the thickness of the seal 10, that is, along surfaces 14. At best, surfaces 14 are most intimate when the cut ends are perfectly aligned. The slightest misalignment, which is bound to take place under the dynamic conditions that the seal is subjected to, reduces the intimacy the contact and thus provides a path for leakage of a fluid under pressure.
The second method involves a slicing dynamic, as shown in FIG. 2. A ring seal, shown generally at 20, also has a continuous circumference before it is cut by cutting tool 22a, 22b , 22c, as shown in three positions during cutting. The cutting motion begins at the position shown by blade 22a as it enters into the seal 20. The seal is cut through to provide chamfered surfaces 24, also shown by cross hatching, and the cutting tool 22c shows the position it would occupy at it emerges from the seal 20.
While slicing does not compress the material of ring seal 20, it does no eliminate tearing. Moreover, a phenomenon associated with tearing both with the plunging method and the slicing method exasperates the requirement of exact alignment of the cutting surfaces in order to maintain the integrity of the seal 20.
As may be seen in FIG. 3, there is a ring seal 30 cut by either of the just described methods. Ring seal 30 has two cut surfaces 32 and 34, which result from the cutting tool cutting through ring seal 30. The edges of the cut surfaces 32 and 34 do not project along the radius of seal 30, but curve away from the axis of the seal so as to result in scalloped edges 36, 38, 40 and 42. The phenomenon of the scalloped edges is associated with all cut lines making up the cut surfaces 32, 34. Thus, a slight misalignment of the two cut surfaces 32 and 34 would cause relative angular displacements of the cut lines making up cut surfaces 32 and 34 so that the surfaces 32 and 34 are not in intimate contact.
In FIG. 1, as blade 12a contacts seal 10, as a point of tendency with respect to the outer diameter of seal 10, and then is plunged into the seal, the cut surfaces 14 expand upwardly and downwardly along the chamfered surface as the cutting edge of the blade becomes an increasing cord within the outer diameter until blade 12b emerges from the inner diameter of seal 10. Accordingly the scalloped edge of the cutting surfaces is duplicated top and bottom with plunge slice. In FIG. 2, blade 22a traverses a different path with respect to its entering both top and bottom. The slicing of seal 20 is in an arc which causes the blade to move in an arc. Thus, the tearing occurs over the arc which explains an exaggerated scalloped edge that is different both top and bottom.
When the cut surfaces are separated so that the seal may be placed over a coaxial part, the seal is likely not to be situated so that it attains its original diameter as it straddles apart. Even if it attains its original diameter when placed in its work position, it will move and expand under the dynamics of its environment. Accordingly, the cut surfaces will ramp upon one another. But as the cut surfaces only match when returned to their original position, there develops a passage for leakage.